1. Field of the Invention
The present invention relates to power distribution systems. More particularly, the present invention relates to an automated power distribution system which incorporates a switching network responsive to control signals to fault isolate overcurrent events and minimize power outages.
2. Description of the Related Art
Electrical power distribution systems generally comprise substations having multiple circuits which have the capability to distribute three-phase electrical power to residential and/or industrial locations. Approximately 1500 customers are typically connected to each circuit. Each phase of each circuit is protected from an overcurrent event by a circuit breaker which is typically located at the substation. Examples of an overcurrent event include lightning striking the electrical conductors creating a current surge on one or more feeder conductors of the circuit, animals on the feeder conductors which create a short circuit, and actual short circuits. When an overcurrent is detected by the substation circuit breaker, the breaker initially actuates an "instantaneous trip and close" of the circuit breaker switch.
As shown in the voltage waveform of FIG. 1, the instantaneous trip of the circuit breaker switch (i.e., the switch opens) occurs at time t.sub.1, when the overcurrent (or fault) is detected. At time t.sub.2, the circuit breaker switch closes in an attempt to correct the cause of the overcurrent. For example, if an animal is on the conductors causing a short circuit, the animal may be dislodged from the conductors when the breaker trips and is closed to turn power back on. In the event the overcurrent was instantaneous, e.g., caused by lightning, then power will be restored to the circuit when the breaker closes. Typically, the circuit breaker switch closes within about four and six cycles of detection of the overcurrent, i.e., time t.sub.2 occurs between about 66 ms and about 100 ms after time t.sub.1
If the cause of the overcurrent is still present at the time the circuit breaker switch is closed, the circuit breaker switch again trips at time t.sub.3. At this time the circuit breaker enters a "first time delay" mode wherein the circuit breaker switch trips for approximately 30 seconds. At time t.sub.4 the breaker switch is again closed. If the cause of the overcurrent is still present, the circuit breaker switch again trips at time t.sub.5 and the circuit breaker enters a "lockout" mode, wherein the cause of the overcurrent must be ascertained before the circuit breaker switch can be closed. In the above configuration, when the circuit breaker is in the lockout mode, every customer connected to the circuit is without electrical power. As a result, it is desirable to isolate the fault and restore power to as many customers as possible connected to the circuit in a relatively short period of time.
One attempt to restore power to customers on a circuit has been to install automatic circuit reclosure (ACR) switches on the utility poles which support the feeder conductors. The ACR switches operate in a similar fashion as the circuit breaker, i.e., the switch opens when an overcurrent is detected. However, the ACR switches have similar operating characteristics as the circuit breaker, thus operating characteristic mismatches often occur between the operation of the ACR switches and the circuit breakers. To illustrate, if an ACR switch and the circuit breaker are designed to trip at a current of 200 amps, and the sensitivity of the circuit breaker is greater than the sensitivity of the ACR switch, then an overcurrent on the conductors would cause the circuit breaker to trip before the ACR switch, thus, circumventing the intended purpose of installing ACR switches. Moreover, since ACR switches trip when current is flowing through the switch, they must be constructed to withstand the arcing and high temperatures which occur between the switch contacts when opening and closing. This construction increases the cost of each ACR switch, rendering them highly uneconomical for usage in large quantities as required to protect each phase of each substation circuit.
Another attempt to fault isolate the cause of the overcurrent is to install multiple normally closed switches in series in each circuit on the utility poles. When an overcurrent is detected by the circuit breaker, each switch is opened and the circuit breaker is then closed. If no overcurrent is detected then each of the switches is sequentially closed until the circuit breaker trips, thereby causing the entire circuit to lose power again and causing damage to the line conductors and associated electrical equipment by closing the breaker into faults.
Therefore, a need exists for a power distribution system which utilizes a switching network that operates when the circuit breaker opens to avoid the arcing and high temperature problems associated with the opening and closing of switches when current flows therethrough and which quickly isolates overcurrent faults and restores power to at least a portion of the customers connected to the circuit.